Objective: To investigate the relationship between alcohol use and unsafe sexual behaviour. Methods: The paper discusses data collected from 461 gay and bisexual men interviewed in England and Wales by Project SIGMA during 1991-2. These data were collected during face-to-face interviews using retrospective weekly diary techniques and include details of all sexual sessions and alcohol use. The 819 reported sexual sessions with other men are divided into those that involved alcohol use (30.6%) and those that did not. Results: Differences in the incidence of HIV risk behaviours between sexual sessions that involve alcohol use and those that did not are small, and none are statistically significant. Furthermore, for those men who engaged in sexual behaviour whilst under the influence of alcohol the quantity of alcohol consumed had no effect on sexual behaviour. Conclusions: Among gay and bisexual men, sex under the influence of alcohol is no more likely to be unsafe than sex among men who have not consumed alcohol. Key words: Gay men; sexual behaviour, alcohol use, unsafe sex, anal intercourse, condom use. P Weatherburn, PM Davies, FCI Hickson, AJ Hunt, APM Coxon, TJ McManus